The number of cars in cities continues to grow, placing an ever-increasing demand on a limited infrastructure of parking locations and roads. However, an appreciable fraction of these cars are in use only a relatively small fraction of the time. Drivers routinely commute without a car by walking, bicycling or by public transportation, and their cars remain idle most of the time. Still others may commute by car. Such cars make traffic worse and occupy a valuable parking spot most workdays.